PTC (positive temperature coefficient [of resistance]) thermistors are electrical components whose primary feature is that their resistance increases in a controlled fashion as the temperature increases above some threshold. A plotted graph of a PTC's resistance and temperature is commonly referred to as an R/T curve. The threshold temperature above which the PTC's resistance increases rapidly is referred to as the Currie Temperature, and exhibits a distinctive transition in the PTC's R/T curve. Before the Currie Temperature, the resistance may be unchanging, or even decline very slightly, but as the Curie temperature is exceeded, the slope of increasing resistance typically becomes very steep.
PTC thermistor devices in many physical configurations are well known in the art (see References below), and have several uses including: (1) as a temperature sensor, (2) as heating elements, and (3) as temperature regulation devices against over-temperature or over-current.
(1) As a temperature sensor—When either an NTC (negative temperature coefficient) or a PTC (positive temperature coefficient) thermistor is used as a temperature sensor, the local environment temperature affects the thermistor's electrical resistance characteristic which can then be monitored by another electronic circuit. NTC thermistors reduce in electrical resistance as temperature increases, and PTC thermistors increase in electrical resistance as temperature increases. A thermistor applied as a sensor may be used to detect whether a temperature limit in equipment, liquids, or other materials is exceeded. Thermistors used as sensors typically have the advantages of small dimensions, low cost, simple reliability, and high control accuracy.
(2) As heating elements—PTC thermistors have also been used in prior art directly as heaters. PTC thermistors are well suited to use as heating elements due to their specific property of increasing resistance as their temperature increases. This property tends to prevent PTC heaters from over-heating and may allow PTC thermistors in some heating applications to be used without other temperature control and regulating components, and some heating applications without requiring over-temperature protection devices. PTC thermistor heating elements have been used when space is a consideration, when high-reliability is desired, when a fail-safe design is required, and wherever measurement and regulating equipment as well as heating devices must be enclosed in small spaces.
(3) As protective devices against over-temperature or over-current—PTC thermistors may be used instead of thermal-cutoff-fuses or conventional current-fuses to protect against over-temperature conditions or over-current loads in motors or other electronic circuits, by placing the PTC electrically in series with the circuit that is to be protected. In an over-current condition, the increased current to the protected circuit causes increased heat dissipation in the PTC Thermistor, and as the PTC Thermistor's temperature increases, its resistance increases. As the PTCs resistance increases, the current to the protected circuit is reduced, which rapidly reduces the power dissipated in the protected circuit, potentially preventing an over-current condition in the protected circuit (eg.: motor or heater). PTC Thermistors thus are capable of limiting the power dissipation of the overall circuit by increasing their resistance, which reduces the current flowing in the protected system. Power dissipation is a product of the resistance times the square of the current, so reducing the current, a squared term, reduces the power dissipation faster than the increasing resistance can increase the power dissipated.
Thermal-cutoff-fuses may also be used for protection where an over-current condition causes over-temperature, or where a heater might be damaged if a power regulation system failure allows the heater to overheat, but PTC thermistors have several advantages over thermal fuses or current fuses. PTCs do not have to be replaced after elimination of the fault but can resume their protective function immediately upon removal of the overload condition, with some time allowed for the PTC to cool.
Because a PTC thermistor can recover from a momentary over-temperature condition, their protected temperature may be selected to be closer to normal operating temperatures without incurring serious consequences from nuisance trips. If a thermal cutoff fuse or current fuse reaches its fuse temperature or current, the fuse “opens” in a “destructive” manner and must be replaced, resulting in the intervention of a repair service call or product return. Because of this, destructive fuses will typically be selected at temperatures that allow larger temperature margins above the normal operating temperature. There are “bimetallic” thermal cutouts which also offer non-destructive operation, but these may be more expensive, may be slower to act due to packaging and size characteristics, and may require manual intervention, or cycling to a much lower temperature than the trip point, in order to be reset. In contrast to this, PTC thermistors can return to their initial resistance value immediately upon cooling below their Curie temperature, even after frequent heating and cooling cycles.
In some cases, the flat disk form of ceramic disk PTC thermistors allows them to have a large surface area of thermal bond with the protected system. This promotes improved thermal conductivity compared to a more conventional thermal fuse package in which the temperature-sensitive element is typically packaged in an enclosure with the fuse element more thermally isolated from the protected system. This improved thermal conductivity of ceramic disk PTC thermistors allows them to more closely and more quickly follow the temperature of the protected system, allowing faster and more accurate protection.
PTC thermistors of prior art are made of various materials, including both ceramic and polymer base substances with various doping additives which promote the PTC resistance effect.
The present invention relates specifically to PTC thermistors in the form of a ceramic disk approximately the size of a coin, with metalized opposing flat surfaces to which electrical connections can be attached. The resistance value in this device is measured between the opposing flat surfaces, the PTC resistance material being sandwiched between the two metalized flat surfaces.
The PTC effect typically relies upon a phase change in the structure of the composite resistance material, changing from a more crystalline structure to a more amorphous structure at what is known as the Curie temperature. This phase change characteristic is typically responsible for increasing the electrical resistance of the composite material. This phase change is also characterized by significant mechanical dimension changes, measured as the CTE (coefficient of thermal expansion) of the material. This CTE expansion is typically greatest above the Curie temperature where the material becomes more amorphous, and is less pronounced below the Curie temperature where the material is more crystalline in structure.
As a result of these CTE dimension changes, in prior art it has been recommended that large ceramic disk PTC devices suitable for high powered applications should not be attached to a substrate by soldering. Quoting an application note entitled “Mounting Instructions,” from one PTC manufacturer:                “ . . . for applications involving frequent switching and high turn-on power. Soldering is not allowed for such applications in order to avoid operational failure . . . ”. (Epcos, 2006c, p. 7)This is at least partly because a solder chosen to have a melting temperature above the operating temperature of the protected system, would freeze into solid form well above the Curie temperature of the PTC, where the PTCs CTE changes are quite large. Then as the PTC and substrate are allowed to cool, the PTC and substrate would exhibit very different CTE changes while attached with a rigid frozen solder joint. The assembly would then come under severe shearing stresses and other stresses which typically will crack the PTC ceramic material, or cause a failure of the solder joint adhesion to one or both surfaces. Smaller PTC devices designed for low power operation may be effectively soldered by carefully following the manufacturers recommendations, and wires may be successfully soldered to the surface of larger high-powered PTC devices, because the soldered area can be quite small, which results in reduced CTE-induced stresses.        
While ceramic disk PTC thermistors have several known uses, the novel ceramic disk PTC attachment structure and method of this invention will be described in reference to use in solid ink marking apparatus. This description is but one example of a use of this invention, provided as an example for clarity, and it is to be understood that the present invention can be used in any suitable system, both presently known and unknown to achieve some or all of the beneficial effects described in this example system. PTC thermistor uses that can benefit from this attachment method include usage as a sensor, as a heating element, or as protective devices against over-temperature or over-current, or with other ceramic electronic components where a mismatched CTE between the device and the substrate it is attached to might prevent the device from being soldered without the novel method described herein.
Solid Ink marking technology employs an ink material which remains in a solid form, technically “frozen” solid at room temperature, but when heated sufficiently changes phase from its frozen solid state to a melted liquid form which can then be manipulated in various ways as any liquid ink to form images on paper. Solid ink marking technology addresses key user requirements, expectations and human factor issues by how it works. Its excellent image creation method, simplicity, and ease of use set it apart from other printer marking methods. Because the ink material is frozen in a solid state at normal human-comfort room temperatures, the packaging and handling is simplified, being not prone to messy handling or spills, and requiring less complicated and wasteful packaging materials which would need to be recycled or disposed of. When the solid ink stick has all been melted and used for printing, there is no container or cartridge left behind in the printing system that must be removed and recycled or disposed of.
Moreover, Solid Ink offers remarkable print quality on the broadest range of print media including cardstock, envelopes and transparencies as well as recycled paper, coated or uncoated paper stocks, and custom page sizes. For example, solid ink printers can accommodate media from 16 lb. bond to over 80 lb cover cardstock. Laser printers vary and can be limited to 58 lb. paper stock. Wet inkjet printers generally require specially treated media which prevents the liquid ink from “bleeding” into the fibers of the paper which causes blurred images, unintended mixing of colors, as well as warping and wrinkling of the paper due to the fibers becoming unstable when wet. Solid ink printer marking does not require coated papers because it uses an ink that turns solid upon contact with the paper and is not subject to these effects. Coated papers for inkjet printing are not always available in a wide range of thicknesses, textures, colors and sizes, and may be more costly.
Solid ink printers are also easy to use and maintain. Ink loading is simple—each color has a unique shape-coded and numbered ink stick which ensures there is no mix up. The right color goes only in the right place and, because solid ink is solid, not wet or powdered, there is no mess. The only other consumable required in a solid ink printing system is a maintenance kit which takes less than a minute to replace, about once a year.
Solid ink has the critical property of remaining in solid form until heated to a very specific temperature whereupon it changes phase from solid to liquid then instantly changes back to solid when allowed to cool upon contact with the paper media. This required control of ink temperatures requires precision heating devices with suitable temperature monitoring, control, and over-temperature protection.
Solid ink is applied through a precise heated print head with tiny holes smaller than a human hair. It uses many ink nozzles jetting more than 30 million drops per second of melted liquid ink. Years of investment, research and experience have yielded multiple generations of inks and heated print heads that work together as a system.
The ink is jetted from the print head to a heated drum where it is maintained at the phase-change temperature of the ink, not liquid, but not fully solid, in a malleable state that ensures precise transfer to the paper. This reduces the amount of ink that can wick into the paper fibers and controls dot spread or image smearing or bleeding.
Precision temperature management is necessary for successful solid ink printing, and heaters may be controlled or protected by PTC thermistors.
More specifics on solid ink printing can be obtained from the public web site www.xerox.com which is incorporated by reference into this disclosure. (Xerox, 2007)